页岩气压裂井瞬态温-压耦合对套管应力的影响

Influence of Transient Thermo-pressure Coupling on Casing Stress for Shale Gas Fracturing Wells

页岩气井压裂过程中,压裂液通过大排量方式注入井筒,井底温度会急剧降低,同时高泵压也增加套管受力,加剧套管失效风险。鉴于此,建立了压裂过程中套管-水泥环-地层组合体瞬态温-压耦合模型,着重分析施工排量、注入温度和施工压力对套管应力的影响。研究结果表明:流变参数会影响对流换热系数,继而影响井底温度的变化,应该选择合理的流变参数,改善压裂液与井筒之间的对流换热;排量增加会迅速降低井底温度,增大套管应力,且原始储层温度越高,温度降低幅度也越大,套管应力增加越多;套管应力随压裂液注入温度降低而增加;合理的施工压力有助于降低套管应力。因此合理的施工泵排量、压裂液注入温度以及施工压力,能有效减小压裂过程中井底温度差,从而改善套管受力,保证压裂过程中套管的安全。

During the fracturing in shale gas wells,the fracturing fluid pumped into the wellbore with large displacement will drastically reduce the downhole temperature. At the same time,the high pump pressure will increase the casing stress and increase the risk of casing failure. In view of this,a transient temperature-pressure coupled model of casing-cement sheath-formation for fracturing is established to emphatically analyze the influence of displacement,injection temperature and pump pressure on casing stress. The research results show that the rheological parameters could affect the convection heat transfer coefficient and then has an effect on the bottomhole temperature. Therefore,rheological parameters should be reasonably selected to improve the convective heat transfer between the fracturing fluid and the wellbore. The increase of displacement could rapidly reduce the bottomhole temperature and increase the casing stress. Higher original reservoir temperature would lead to greater bottomhole temperature reduction and greater casing stress increase. The casing stress increases as the fracturing fluid injection temperature decreases. Proper pumping pressure could help to reduce casing stress. proper pump displacement,fracturing fluid temperature and pumping pressure can effectively reduce the bottomhole temperature difference during fracturing,thus improving the casing stress and ensuring the casing safety in fracturing.